The present invention relates generally to the mechanical processing of chemical products, and, more particularly, to a process for preparing uniformly sized granules of fertilizer materials which do not go through a thermoplastic temperature range in normal granulation processing.
Ammonium sulfate is a commonly available industrial and agricultural chemical typically prepared by reacting ammonia and sulfuric acid in a crystallizer. Unless the crystallization conditions are closely controlled, the ammonium sulfate may be formed as very fine crystals which do not readily blend and distribute evenly with other fertilizer ingredients, and cannot be readily distributed with spreaders. This fine crystalline form presents a problem to the fertilizer industry. Ammonium sulfate is often desirably distributed in a mixture with other fertilizer chemicals, and the ammonium sulfate in fine crystalline form does not spread evenly in such a mixture.
Several approaches have been proposed to overcome this problem, including the granulation of ammonium sulfate with the addition of a chemical serving as a granulation aid such as ammonium phosphate to induce adherence of the fine crystals. However, the commonly available granulation aids have been found to be difficult to use to produce a consistently sized product, in that large quantities must be added to essentially encapsulate the ammonium sulfate within the aid, as there is little tendency for the aid to stick to the sulfate particle. Such consistent sizing of the final granules is highly desirable, inasmuch as recycling of undersized or oversized materials in the preparation plant is minimized, and the ability to mix the ammonium sulfate granules with other fertilizer ingredients is improved.
Although ammonium sulfate has been used as a specific example, this particular fertilizer material is representative of a class of fertilizer materials which exhibit comparable properties during preparation. Such fertilizer materials do not themselves experience a thermoplastic zone at elevated temperatures during their processing. Consequently, the small crystals or particles of such fertilizer materials cannot be made to adhere to each other naturally to form larger granules during processing, unless a granulation aid is added. Examples of such fertilizer materials include potassium sulfate, gypsum, lime and limestone, iron salts such as sulfates, magnesium sulfate and carbonates, sulfur, various metal chelates, and combinations thereof.
Accordingly, there has been a continuing need for a process to prepare a commercially usable form of ammonium sulfate, and similar materials which do not naturally agglomerate during processing. Preferably, the process would result in uniformly sized granules, whose size may be controlled through adjustment of process parameters. The present invention fulfills this need, and further provides related advantages.